Constrained motion bone screw assembly

ABSTRACT

A bone screw assembly includes an anchor portion and a head portion, such as a rod-receiving portion, movably mounted to the anchor portion to allow for controlled angulation between the anchor portion and the head portion. The anchor portion is pivotable in one or more selected directions about an axis relative to the head portion. A restriction member, which may be a rod seat, prevents the anchor portion from pivoting in one or more different directions about another axis relative to the head portion and/or a spinal fixation element received in the head portion. The restriction member may be inserted in the head portion to control direction that the anchor portion pivots relative to the head portion. The restriction member may also serve as a compression member and/or rod seat for seating a spinal rod coupled to the bone screw assembly.

RELATED APPLICATIONS

The present application is a continuation application of U.S.application Ser. No. 16/217,329 filed on Dec. 12, 2018, entitled“Constrained Motion Bone Screw Assembly,” which is a continuation toU.S. application Ser. No. 15/711,380 (now U.S. Pat. No. 10,172,648)filed on Sep. 21, 2017, entitled “Constrained Motion Bone ScrewAssembly,” which is a continuation to U.S. application Ser. No.14/754,259 (now U.S. Pat. No. 9,795,416), filed on Jun. 29, 2015,entitled “Constrained Motion Bone Screw Assembly,” which is acontinuation to U.S. application Ser. No. 11/073,325 (now U.S. Pat. No.7,951,172) filed on Mar. 4, 2005, entitled “Constrained Motion BoneScrew Assembly,” the contents of which are hereby incorporated herein byreference in their entireties.

FIELD OF THE INVENTION

The present invention relates to spinal fixation devices used inorthopedic surgery. More particularly, the present invention relates toa bone screw for coupling a spinal rod to a bone, such as the pedicle.

BACKGROUND OF THE INVENTION

Spinal fixation systems may be used in surgery to align, adjust and/orfix portions of the spinal column, i.e., vertebrae, in a desired spatialrelationship relative to each other. Many spinal fixation systems employa spinal rod for supporting the spine and for properly positioningcomponents of the spine for various treatment purposes. Vertebralanchors, comprising pins, bolts, screws, and hooks, engage the vertebraeand connect the supporting rod to different vertebrae. The size, lengthand shape of the cylindrical rod depend on the size, number and positionof the vertebrae to be held in a desired spatial relationship relativeto each other by the apparatus.

Spinal fixation elements can be anchored to specific portions of thevertebra. Since each vertebra varies in shape and size, a variety ofanchoring devices have been developed to facilitate engagement of aparticular portion of the bone. Pedicle screw assemblies, for example,have a shape and size that is configured to engage pedicle bone. Suchscrews typically include a threaded shank that is adapted to be threadedinto a vertebra, and a head portion having a spinal fixationelement-receiving element, which, in spinal rod applications, is usuallyin the form of a U-shaped slot formed in the head portion for receivingthe rod. A set-screw, plug, cap or similar type of closure mechanism isused to lock the rod into the rod-receiving portion of the pediclescrew. In use, the shank portion of each screw is then threaded into avertebra, and once properly positioned, a fixation rod is seated throughthe rod-receiving portion of each screw. The rod is locked into place bytightening a cap or similar type of closure mechanism to securelyinterconnect each screw and the fixation rod. Other anchoring devicesalso include hooks and other types of bone screws.

Monoaxial screws are a type of screw in which the longitudinal axis ofthe threaded shank is fixed relative to the head portion, or rod slot.The longitudinal axis of the threaded shank may be aligned with thelongitudinal axis of the head portion, and/or the threaded shank extendsat a fixed angle relative to the head. In fixed pedicle screws, whichare used in the pedicle region of the vertebra, the threaded shank isrigidly connected to or integrally formed with the head such that theorientation of the threaded shank is fixed with respect to the head.

Polyaxial pedicle screws have been designed to allow angulation of oneportion of the screw relative to another portion of the screw and thespinal fixation element coupled to one portion of the screw. Forexample, polyaxial pedicle screws allow for a shaft portion to pivotrelative to a rod-receiving portion in all directions about a 360° arearound the rod-receiving portion. Polyaxial screws may be useful forpositioning bone anchors on adjacent vertebrae, when the close proximityof adjacent vertebrae can result in interference between the boneanchors. Polyaxial screws allow for pivoting of the screws in anydirection out of alignment with each other to avoid such interference.

An example of such a polyaxial pedicle screw assembly is described indetail in U.S. Patent Application Publication Number US 2004/0186473entitled “Spinal Fixation Devices of Improved Strength and Rigidity”,U.S. Patent Application Publication Number US 2004/0181224 entitled“Anchoring Element for Use in Spine or Bone Surgery, Methods for Use andProduction Thereof” and U.S. Patent Application Publication Number US2003/0100896, entitled “Element With a Shank and a Holding ElementConnected to It for Connecting to a Rod”, the contents of which areherein incorporated by reference.

Polyaxial and multi-axial screws, which allow the screw shank to pivotin all directions about the head portion, can be difficult to controland often result in movement of the screw shank in planes in whichmovement is not desirable. For example, during vertebral body rotationmaneuvers, which require application of force to the screw head, it isnot desirable for the screw shank to move relative to the screw head.

SUMMARY OF THE INVENTION

The present invention provides a bone screw assembly that provides forcontrolled movement between an anchor portion and a rod-receivingportion of the bone screw assembly. The bone screw assembly allows theanchor portion to pivot about the rod-receiving portion and/or a spinalfixation element received in the rod-receiving portion in one or moredirections, while limiting the movement in other selected directions.For example, the anchor portion can pivot about a first axis that passesthrough the head of the anchor portion and is perpendicular to alongitudinal axis of a rod received in the rod-receiving portion, sothat the anchor portion aligns with the rod in a selected plane, whilebeing restricted from rotation about one or more other axes of the head.When assembled in a patient, the anchor portion may be moveable in atleast one plane, such as the coronal plane, to allow for movement ofvertebral bodies coupled to the rod by the bone screw assembly in one ormore selected directions, while fixed in at least one plane, such as thesagittal plane, to prevent movement in one or more other directions.

According to a first aspect of the invention, a bone anchor assemblycomprises a bone anchor having a proximal head and a distal shaftextending along a longitudinal axis configured to engage bone, areceiving member for receiving a spinal fixation element and forengaging the proximal head of the bone anchor and a restriction memberinserted in the receiving member. The restriction member allows the boneanchor to pivot relative to the receiving member about a first axis ofthe proximal head in at least a first direction and restricts the boneanchor from pivoting about a second axis of the bone anchor in a seconddirection. The proximal head may be received in a cavity of thereceiving portion.

In one embodiment, the first axis is perpendicular to a longitudinalaxis of the spinal fixation element. The second axis may be parallel toa longitudinal axis of the spinal fixation element.

The proximal head may have a first curved side surface to facilitatepivoting of the bone anchor in the first direction. In one embodiment,the proximal head includes two opposed side surfaces that are curved.One or more of the curved side surfaces may be curved in threedimensions.

The proximal head may have at least one flat side surface to preventpivoting of the bone anchor in the second direction.

According to one embodiment, the bone anchor is pivotable in a coronalplane when inserted in a patient. According to one embodiment, the boneanchor is fixed from moving in a sagittal plane when inserted in apatient.

The restriction member in the bone anchor assembly according to thefirst aspect of the invention may comprise a cap for seating theproximal head within a cavity of the receiving member. In oneembodiment, the cap includes a seat for receiving the spinal fixationelement. The cap may also include a first protrusion for guiding themovement of the bone anchor, which may be coupled to or integrallyformed with the cap. The proximal head may include a projection or othersuitable mating means for mating with a recess on the protrusion. Asecond protrusion may extend from the cap, with each protrusionconfigured to abut a side surface of the proximal head. One or both ofthe protrusions may have a flat surface configured to abut acorresponding flat surface of proximal head.

The anchor portion may be restricted to pivoting about a single axisonly relative to the rod-receiving portion, or may pivot about multipleaxes relative to the rod-receiving portion.

According to another aspect, a bone anchor assembly comprises a boneanchor having a distal shaft extending along a longitudinal axisconfigured to engage bone and a proximal head having at least one flatside surface extending substantially parallel to the longitudinal axisand a restriction member. The restriction member receives the proximalhead on a first side and configured to mate with a first flat sidesurface of the proximal head to prevent pivoting of the distal shaftabout a first axis of the proximal head that is parallel to the firstflat side surface.

According to another aspect of the invention, a bone anchor assemblycomprises a bone anchor having a distal shaft extending along alongitudinal axis configured to engage bone and a substantiallyspherical proximal head having at least one flat side surface extendingsubstantially parallel to the longitudinal axis. A receiving memberreceives a spinal fixation element and movably engages the sphericalproximal head. A restriction member is inserted in the receiving memberfor mating with the flat side surface of the anchor head to preventrotation of the bone anchor relative to the receiving member in adirection that is perpendicular to the flat side surface.

The restriction member in the bone anchor assembly may comprise a capdisposed over a top surface of the proximal head and a first protrusionextending from the cap over the flat side surface of the proximal head,and the first protrusion may be integrally formed with or coupled to thecap. In one embodiment, the first protrusion has a flat surfaceconfigured to abut the flat surface of proximal head. The restrictingmember may further comprise a second protrusion opposed to the firstprotrusion for mating with a second flat surface on the proximal head.

According to still another aspect of the invention, a bone anchorassembly comprises a bone anchor having a distal shaft extending along alongitudinal axis configured to engage bone and a proximal head havingat least one flat side surface extending substantially parallel to thelongitudinal axis and a capping member configured to engage the proximalhead on a first side and a spinal rod on a second side, the cappingmember including a first protrusion extending over and abutting a firstflat side surface to prevent rotation of the distal shaft about a firstaxis of the proximal head that is parallel to the first flat sidesurface. The capping member may prevent the anchor portion from pivotingout of a plane aligned with the rod. The proximal head may besubstantially spherical in shape.

The first flat side surface on the proximal head may include aprojection for engaging a recess in the protrusion to mate the firstflat side surface to the protrusion.

The capping member may include a rod seat for receiving the rod on thesecond side. The rod seat may have a longitudinal axis that isperpendicular to the first axis of the proximal head about which theshaft pivots.

According to another aspect of the invention, a bone anchor assemblyincludes a bone anchor having a distal shaft extending along alongitudinal axis configured to engage bone and a proximal head and arod seat coupled to the bone anchor for seating a spinal rod. The rodseat allows for a relative pivoting movement between the bone anchor anda spinal rod inserted in the rod seat in at least a first direction,while restricting relative pivoting movement between the bone anchor andthe spinal rod in a second direction. The rod seat may pivot relative tothe bone anchor to facilitate pivoting of the bone anchor relative tothe spinal rod. The bone anchor assembly may further comprise areceiving member coupled to the bone anchor for housing the rod seat.The rod seat may comprise a lower rod seat coupled to the receivingmember and having a substantially spherical surface configured toslidably mate with a recess in the proximal head of the bone anchor andan upper rod seat pivotably connected to the receiving member, so thatthe lower rod seat and upper rod seat define therebetween a movablechannel for receiving the spinal rod and for allowing relative movementbetween the bone anchor and the spinal rod.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of theinvention will be apparent from the following description and apparentfrom the accompanying drawings, in which like reference characters referto the same parts throughout the different views. The drawingsillustrate principles of the invention and, although not to scale, showrelative dimensions.

FIG. 1 is a diagram of the human body, illustrating the three planesused to help describe the human anatomy.

FIG. 2 illustrates a constrained motion bone screw assembly according toan embodiment of the invention.

FIG. 3 is a top view illustrating the area in which the anchor portionof the bone screw of FIG. 2 is movable relative to the head portionaccording to one aspect of the invention.

FIG. 4 is a top view illustrating the area in which the anchor portionof the bone screw of FIG. 2 is movable relative to the head portionaccording to another aspect of the invention.

FIG. 5 illustrates an embodiment of a constrained motion bone screwaccording to an illustrative embodiment of the invention.

FIG. 6 is a side view of the assembled bone screw of FIG. 5 .

FIG. 7 is a cross-sectional view along axis A-A of FIG. 6 .

FIG. 8A-8C illustrate in detail the compression and restriction memberof the bone screw assembly of FIG. 5 .

FIG. 9A-9C are detailed views of the constrained motion bone screwassembly of FIG. 5 in the vicinity of the receiving member.

FIG. 10 illustrates a constrained motion bone screw according to anotherembodiment of the invention.

FIG. 11 illustrates a constrained motion bone screw according to anotherembodiment of the invention.

FIG. 12 illustrates a constrained motion bone screw according to anotherembodiment of the invention.

FIGS. 13A-13C illustrate a constrained motion bone screw according tostill another embodiment of the invention.

FIGS. 14A-14B illustrate different views of a rod seat for theconstrained motion bone screw of FIGS. 13A-13C.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides an improved bone screw assembly in aspinal fixation system. One skilled in the art will recognize that theinvention is not limited to use in bone or in spinal surgery, and thatthe instrument and methods described herein can be adapted for use withany suitable surgical device to be moved into a selected position in avariety of medical procedures. The present invention will be describedbelow relative to certain exemplary embodiments to provide an overallunderstanding of the principles of the structure, function, manufacture,and use of the instruments disclosed herein. Those skilled in the artwill appreciate that the present invention may be implemented in anumber of different applications and embodiments and is not specificallylimited in its application to the particular embodiments depictedherein.

During spinal deformity surgeries, it may be necessary to de-rotate thevertebral bodies to normalize the spine. Due to varying patient anatomy,insertion of fixed angle screws, where the anchor portion of the screwextends at a fixed angle relative to the rod-receiving portion of thescrew can be difficult. Polyaxial and multi-axial screws, which allowthe screw shank to pivot in all directions about the head portion, canbe difficult to control and often result in undesirable movement incertain planes. A constrained motion bone screw assembly, differentembodiments of which are illustrated in FIGS. 3-13C, allows forangulation of the anchor portion relative to a head portion in at leastone plane, such as the coronal plane of the human body, but preventsangulation in another plane, such as the sagittal plane of the humanbody. For a bone screw assembly used to couple a spinal rod to bone,such as the pedicle bone, to prevent angulation in the sagittal plane,the controlled movement bone screw assembly permits rotation of theanchor portion about an axis perpendicular to a rod coupled to the bonescrew assembly, while preventing, blocking, prohibiting or otherwiseconstraining rotation of the anchor portion about an axis extendingparallel to the rod. The controlled movement bone screw assembly of thepresent invention may allow a surgeon to rotate vertebral bodies andfacilitates rod placement into the rod-receiving portion.

The exemplary bone screw assemblies of the illustrative embodiments ofthe invention may be employed to engage one or more spinal fixationelements to bone. For example, a bone screw assembly may be employed tofix a spinal plate, rod, and/or cable to a vertebra of the spine.Although the exemplary bone screw assemblies described below aredesigned primarily for use in spinal applications, and specifically thepedicle region of a vertebra, one skilled in the art will appreciatethat the structure, features and principles of the exemplary bone screwassemblies, as well as the other exemplary embodiments described below,may be employed to couple any type of orthopedic implant to any type ofbone or tissue.

The bone screw assembly described herein facilitates the correction ofthe position, for example, the angular orientation, of the vertebra inwhich the bone screw is implanted. For example, the bone screw assemblymay be configured to provide stability in one plane, for example, thetransverse plane, by restricting pivoting of the receiver member of thebone screw assembly in the selected plane. The stability of the bonescrew assembly in the selected plane facilitates movement of the bonescrew assembly and associated vertebra in the selected plane. e.g.,facilitates rotation of the bone anchor assembly and the vertebra aboutan axis that intersects the plane. Exemplary instruments and methods formanipulating a bone anchor assembly connected to a vertebra aredescribed in detail in U.S. patent application Ser. No. 11/073,352,filed concurrently herewith, entitled Instruments and Methods forManipulating a Vertebra, incorporated herein by reference.

FIG. 1 is a diagram of the human body, and illustrates the three planesused to help describe anatomy. As shown in FIG. 1 , the sagittal plane20 splits the body from head to toe, from the back or posterior to thefront or anterior. The coronal plane 24 splits the body from head totoe, side to side. The transverse plane 26 slices through the bodyyielding cross-sectional views. According to one aspect of theinvention, a constrained motion bone screw assembly 30, shown in FIG. 2, includes an anchor portion 32 that is controllably pivotable inselected directions about one or more axes passing through a pivot pointP with respect to a head portion 34, which may be a receiving portion,such as a rod-receiving portion, for receiving a spinal rod or otherspinal fixation element.

The constrained motion bone screw assembly 30 of the present inventionfurther selectively constrains the movement of the anchor portion 32relative to the head portion 34 and/or a spinal fixation elementreceived in the head portion 34 in one or more selected directions. Asshown in FIG. 2 , the anchor portion 32 relative to the head portion 34in several directions about pivot point P, as indicated by arrows 35a-35 e. The illustrative anchor portion 32 is prevented from pivotingabout axis R-R, which extends through the pivot point P in a directionthat is perpendicular to the longitudinal axis of 22 of the shaft. Theanchor portion 32 is thus restricted from rotating about the headportion 34 in a direction that is perpendicular to the axis R-R.Preferably, the anchor portion 32 may be adjusted such that thelongitudinal axis 22 of the bone anchor portion 32 extends at an angleof between 0° and 90° in the selected direction relative to thelongitudinal axis 42 of the head portion 34.

In an alternate embodiment, a rod seat within the head portion 34 may beselectively movable to allow for relative movement between a spinalfixation element, such as a spinal rod, received in the head portion 34and the bone anchor.

FIG. 3 is a top view illustrating the range of motion of the anchorportion 32 relative to the head portion 34 of a bone screw assembly 30and a rod 12 received by the head portion 34 according to one embodimentof the invention. As shown in FIG. 3 , the path of the anchor portion inthe region surrounding the head portion may be limited in the sagittalplane, but movable in all other planes around the head portion,including the coronal plane. In FIG. 3 , the anchor portion is pivotablenot only about axis T-T, which is perpendicular to the longitudinal axisof the rod 12 received by the head portion 34, but also around severalintermediate axes I1-I4 extending between the perpendicular axis T-T andthe axis R-R to allow movement in the shaded region 40 surrounding thehead portion 34. The anchor portion 32 is fixed from rotating about axisR-R, to prevent movement of the anchor portion in perpendicular region44 extending perpendicular to the longitudinal axis 42 of the headportion.

As shown in FIG. 4 , according to another embodiment of the invention,the anchor portion 32 may be fixed in all planes except for one selectedplane, such as the coronal plane, to allow for movement of the anchorportion along a selected path in a single plane, such as the sagittalplane, while preventing movement of the anchor portion out of the singleplane in all other directions relative to the head portion. In theembodiment of FIG. 4 , the anchor portion is only pivotable about axisT-T, so that the range of motion of the anchor portion relative to thehead portion is limited to the region 40′ that is parallel to andaligned with the rod 12. Because the anchor cannot pivot about otheraxes, the anchor portion cannot move into the region 44′ and is fixed inthe plane defined by region 40′. The region 40′ may encompass thesagittal plane when the screw assembly is inserted in a patient. Oneskilled in the art will recognize that ranges of motion between thoseillustrated in FIGS. 3 and 4 are also contemplated by the presentinvention.

FIG. 5 is an exploded view of an embodiment of a bone screw assembly 100including a bone anchor portion 114, illustrated as a threaded shaft,coupled to a head portion, illustrated as a rod-receiving portion 140,to allow for controlled movement between the bone anchor 114 androd-receiving portion 140 in accordance with the teachings of theinvention. A compression and restriction member 180 for seating the headof the anchor portion 114 within the rod-receiving portion 140 includesrestricting protrusions or other suitable mechanisms for selectivelylimiting the movement of the bone anchor relative to the rod-receivingportion to one or more selected directions. FIG. 6 is a side view of theassembled bone screw assembly 100. FIG. 7 is a cross-sectional view ofthe assembled bone screw assembly 100 along axis A-A shown in FIG. 6 .

The bone anchor 114 comprises a joint portion, illustrated as a proximalanchor head 116, for coupling the bone anchor 114 to the rod-receivingportion 140, and an anchoring portion, illustrated as a distal shaft 118configured to engage bone. The distal shaft 118 of the bone anchor 114has a shaft diameter 120 and a longitudinal axis 122. The distal shaft118 may include one or more bone engagement mechanisms to facilitategripping engagement of the bone anchor to bone. In the illustratedembodiment, the distal shaft 118 includes an external thread 124extending along at least a portion of the shaft for engaging bone. Inthe illustrated embodiment, the external thread 124 is a single leadthread that extends from a distal tip 126 of the shaft to the anchorhead 116, though one skilled in the art will recognize that the externalthread may extend along any selected portion of the shaft and have anysuitable number of leads. Other suitable bone engagement mechanismsinclude, but are not limited to, one or more annular ridges, multiplethreads, dual lead threads, variable pitched threads and/or anyconventional bone engagement mechanism.

The rod-receiving member 140 receives the proximal head 116 of the boneanchor to couple the bone anchor 114 thereto, thereby coupling the boneto a rod or other element received in the rod-receiving member 140. Theillustrative rod-receiving member 140 may be substantially similar to ahead portion of a polyaxial screw assembly of the prior art. In a restposition, the longitudinal axis 122 of the bone anchor aligns with alongitudinal axis 142 extending through the rod-receiving member 140.The distal shaft 118 is pivotable relative to the rod-receiving member140 about the proximal head 116 in one or more selected directions toangulate the longitudinal axis 122 relative to the longitudinal axis142. The screw assembly 100 further includes one or more components,illustrated as the compression and restriction member 180, forpreventing a pivoting movement of the distal shaft 118 in one or moredirections, so that the distal shaft 118 cannot pivot in all 360 degreesaround the rod-receiving member 140, thereby increasing the stability ofthe screw assembly in one or more planes, as described in detail below.For example, referring to FIGS. 6 and 7 , the shaft is pivotable aboutaxis T-T, but constrained from pivoting about axis R-R. Axis R-R isaligned with and parallel to the longitudinal axis r-r of the rod 12 ina selected plane and perpendicular to axis T-T, intersecting T-T atpivot point P, and may be substantially parallel to the longitudinalaxis r-r of a rod to be received in the receiving portion 140.

The anchor head 116 of the bone anchor 114 may be configured tofacilitate controlled adjustment of the bone anchor 114 relative to thereceiving member 140 of the bone screw assembly. For example, theillustrative anchor head 116 may be substantially spherical and includecurved side surfaces 161, 162 that are shaped to permit pivoting of thebone anchor 114 relative to the receiving member 140 in one or moreselected directions. The curved side surfaces 161, 162 are preferablycurved in three-dimensions to facilitate rotation of the anchor portion114 relative to the receiving member 140. The illustrative anchor head116 further includes two opposed flat side surfaces 163, 165 forconstraining the pivoting movement to the one or more selecteddirections. The flat surfaces 163, 165 preferably extend substantiallyparallel to the longitudinal axis 122 of the shaft 118. While theillustrative embodiment shows two opposed flat side surfaces 163, 165,one skilled in the art will recognize that the head can have anysuitable number of flat surfaces or other selected feature for limitingthe path of the shaft 118 relative to the receiving portion 140 aboutany selected axis or axes. The top surface 167 of the anchor head 116may be a generally planar surface to facilitate seating of the anchorwithin the rod-receiving portion 140 of the screw assembly. The anchorhead 116 may also have surface texturing, knurling and/or ridges.

The illustrative bone screw assembly 100 further includes a compressionand restriction member 180 for seating the anchor head 116 within therod-receiving portion 140 of the screw 100 and for cooperating with theflat surfaces 163, 165 to constrain the movement of the anchor portionrelative to the rod-receiving portion 140 and/or a rod received therein.The compression and restriction member 180 preferably forms a proximalrod seat 182 for seating a rod or other spinal fixation element and anopposed distal anchor seat 197 for engaging the anchor head 116. FIGS.8A-8C illustrate an embodiment of the compression and restriction member180 in detail, though one skilled in the art will recognize that theinvention is not limited to the illustrative embodiment. Theillustrative compression and restriction member 180 includes a cap 181and restricting protrusions 192, 194 that extend from a lower surface184 of the cap 181. The restricting protrusions 192, 194 form atrack-like region 197 for receiving the anchor head 116 therebetween.The restricting protrusions 192, 194 are configured to mate with theflat surfaces 163, 165 of the anchor head 116 when the bone screwassembly 100 is assembled to guide and constrain the pivoting movementof the anchor head 116 relative to the receiving member 140. Theillustrative restricting protrusions 192, 194 restrict movement of theanchor head 116 about axis T-T through a plane that is parallel to theflat faces 163, 165 of the proximal head 116 and the protrusions 192,194.

In illustrative embodiment of FIGS. 5 -SC, the plane through which theanchor portion 114 pivots is preferably defined by the longitudinal axisr-r of a rod inserted in the rod-receiving member 140 when the screwassembly 100 is assembled and the longitudinal axis 142 of the receivingmember 140, similar to the assembly of FIG. 4B. However, one skilled inthe art will recognize that the screw assembly 100 of FIGS. 5-8C mayalso be made to pivot in one or more other directions relative to therod-receiving member 140.

In the embodiment shown in FIGS. 8A-8C, the restricting protrusions 192,194 comprise separate inserts that couple to the cap 181. For example,the illustrative cap 181 includes side recesses 182 a, 182 b, each sizedand configured to receive a top end of a restricting protrusion 192,194, respectively. Each recess may further include a coupling projection183 a. 183 b configured to mate with a hole or recess 195 a, 195 b, inan associated restricting projection 192, 194, respectively, tofacilitate coupling of the restricting projection to the cap. Whencoupled, each restricting projection extends past the bottom surface 184of the cap 181 to cover and abut the flat surfaces 163, 165 of theanchor head 116 when the screw is assembled to control the movement ofthe anchor 114 relative to the rod-receiving member 140.

One skilled in the art will recognize that any suitable means forcoupling the restricting protrusions to the cap 181 may be used.Alternatively, one or more of the restricting protrusions 192 or 194 maybe integrally formed with the cap 181.

The restriction and compression member 180 is positioned within thereceiving member 140 between the spinal rod 12 and the anchor head 116when the bone screw assembly is assembled. The restriction andcompression member 180 preferably engages the spinal rod 12 and theanchor head 116 to facilitate assembly of the constrained motion bonescrew assembly 100.

According to another embodiment of the invention, a restriction memberis provided for restricting pivoting of the bone anchor relative to thereceiving member that does not necessarily serve as a compression memberand/or a rod seat for seating the spinal rod or other spinal fixationelement coupled to the bone anchor assembly.

FIGS. 9A-9C are detailed views of the illustrative rod-receiving member140 when assembled. As shown in FIG. 9B, the flat surfaces 163, 165 ofthe proximal head interact with the flat surfaces of the protrusions192, 195 to prevent movement or rotation of the shaft 118 against theflat surfaces. As shown in FIG. 9C, the curved side surfaces 161, 162 ofthe proximal head 116 allow for rotation of the bone anchor 114 relativeto the rod-receiving member 140 in the direction indicated by arrow 90about axis T-T.

In the illustrative embodiment, the restricting protrusions 192, 194restrict the movement of the anchor shaft along a predetermined axisthrough an interference fit between the flat surfaces 163, 165 of theanchor head 116 and the restricting protrusions 192, 194. However, oneskilled in the art will recognize that any suitable means may be used torestrict the movement of the shaft to one or more selected directions.

The invention is not limited to the illustrated mechanism forconstraining the motion of the shaft relative to the rod-receivingportion. For example, as shown in FIG. 10 , the anchor head 116 mayinclude projections 166 a, 166 b extending from the side surfaces 163,165, respectively that are configured to interface with recesses 196 a,196 b in the restricting protrusions 192, 194 of the illustrativecompression and restriction member to form a fixed pivot point aboutwhich the anchor 114 can rotate. Alternatively, the restrictingprotrusions may include projections configured to be received in therecesses formed in the side surfaces 163, 165 to facilitate coupling ofa compression and restriction member to the proximal head thatselectively limits rotation of the anchor in one or more directionswhile facilitating rotation in one or more other directions.

In addition, while the illustrative protrusions include flat surfacesconfigured to abut flat surfaces on the proximal head 116 to restrictrotation along a single axis, the restricting protrusions canalternatively be designed to allow for rotation about one or more of theintermediate axes I1-I4 shown in FIG. 3 .

According to an illustrative embodiment of the invention, the receivingmember 140 of the constrained motion bone screw assembly defines arecess 148. The recess 148 may be sized and shaped to receive a spinalrod 12 that extends along axis r-r or another suitable spinal fixationelement. The exemplary spinal rod 12 may be seated within the recess 148by aligning the spinal rod 12 and the recess 148 and advancing thespinal rod through a top bore hole into the recess 148. Theconfiguration of the recess 148 may be varied to accommodate anysuitable spinal fixation element. A suitable configuration for thereceiving member 140 is described in the U.S. Patent ApplicationPublication Numbers US 2004/0186473, US 2004/0181224 and US2003/0100896, the contents of which are herein incorporated byreference.

In other embodiments, a spinal fixation element may be coupled to thebone anchor by alternative coupling mechanisms in place of a recess,including, for example, an offset coupling mechanism, such as a bandclamp, sacral extender, or a lateral off-set connector.

The receiving member 140 may couple the spinal fixation element seatedtherein to the bone anchor 116 through any suitable means. For example,in the illustrative embodiment, the distal end of the receiving memberincludes an opening 160 through which at least a portion of the boneanchor 114 may extend. The distal opening is preferably smaller in sizeand shape than the anchor bead 116 so as to engage the head 116 of thebone anchor 114. The distal opening 160 may define a seat 169 to allowthe bone anchor 114 to selectively pivot relative to the receivingmember. The screw is assembled by inserting the shaft through the firstopening 160 until the head 116 is received in and constrained by thecavity 169.

The illustrative compression and restriction member cap 181 may begenerally disc-shaped having a circular cross-section or other crosssection preferably corresponding to a first bore 144 of the receivingmember 140. A first surface of the compression and restriction member180 may be configured to seat the spinal fixation element. In theillustrative embodiment, the seat 182 formed in the first surface has agenerally arcuate cross-section having a curvature that may approximatethe curvature of the exemplary spinal rod to be received therein. Thesecond surface 184 may be configured to engage the anchor head 116. Forexample the second surface 184 may have a generally concave sphericalshape or a tapered shape to engage the head of the bone anchor. Theillustrative second surface 184 has a hemispherical shape to approximatethe curvature of the anchor head 116. A bore 186 may extend through thecap 181 to allow for advancement of an instrument to the bone anchor 116during assembly of the bone screw assembly.

After pivoting the bone anchor portion 116 about a selected axis in aselected direction relative to the receiving portion 140 by a selecteddegree, preferably between 0° and 90°, a user can lock the orientationof the anchor portion relative to the rod-receiving portion by insertinga closure mechanism, such as a set screw. The closure mechanism securesa spinal rod 12 or other suitably configured spinal fixation elementwithin the recess 148 of the receiving member 140 and locks the anchorhead 116 in the selected orientation within and relative to thereceiving member 140. In the illustrative embodiment, distal advancementof the closure mechanism into engagement with the spinal rod 12 in therecess 148 seats the spinal rod in the seat 182 of the compression andrestriction member 180. The compression and restriction member 180 orother suitable restriction member may compress against the anchor head116 to lock anchor in the selected orientation. Other suitable closuremechanisms may be employed to secure the spinal fixation element to theassembly and/or to lock the orientation of the bone anchor relative tothe receiving portion.

While the illustrative restricting protrusions 192, 194 restrictpivoting of the anchor in a single direction about a single axis, oneskilled in the art will recognize that the invention is not limited torestricting movement to a single direction about a single axis. Asdescribed above, the compression and restricting member 180 or othersuitable restriction member may also be configured to allow somerotation of the anchor portion about the longitudinal axis 122, or allowpivoting in an intermediate direction about an intermediate axis I-Ibetween axes T-T and R-R, while restricting the anchor from being ableto move in any direction in the full 360 degree around the rod-receivingmember 140, as shown in FIG. 3 .

The receiving member 140, in certain exemplary embodiments, may beconfigured to receive a spinal fixation element, such as a rod, andcouple the spinal fixation element to the bone screw assembly 100. Asshown, the recess 148 is sized and shaped to receive a spinal rod,though one skilled in the art will recognize that the receiving member140 may be configured to accommodate any suitable spinal fixationelement.

In another embodiment of the invention, shown in FIG. 11 , the proximalhead 116′ of the anchor portion 114′ of a constrained motion bone screwassembly 100′ is substantially spherical and curved on all sidesurfaces. The proximal head 116 includes a cavity 1162 formed in the topsurface for receiving a ball end 1165 of a receiving member 140′. Aretention ring 1164 inserted in the cavity 1162 secures the ball end1165 within the cavity to couple the anchor portion 114′ to thereceiving member 140′. A support collar 1168 extends from the receivingmember 140 over a portion of the proximal head 116′ to maintain theposition of the anchor portion and the receiving member. The ball end1165 of the receiving member may include one or more flat surfaces thatabut flat surfaces on the retention ring 1164 to constrict rotation ofthe anchor portion relative to the ball end in one or more selecteddirections.

Alternatively, the collar portion 1168 and the outer surface of thesubstantially spherical proximal head 116′ may be configured so as toselectively prohibit rotation of the anchor portion relative to thereceiving member in one or more selected directions while allowingrotation in one or more different directions.

Other details of the bottom-loading screw assembly shown in FIG. 11 aredescribed in U.S. Pat. No. 6,623,485 which is incorporated herein byreference.

While the illustrative embodiment is a top-loading screw, one skilled inthe art will recognize that the present invention encompasses abottom-loading screw as well. For example, the first opening 160 of thereceiving member may be larger than the head 116 to allow the head topass through the opening 160 during assembly of the screw. The anchorhead would then be inserted through a bottom opening of the receivingmember and retained therein by a securing means, i.e., the anchor headis smaller in diameter than the bottom opening of the receiving member.In contrast, the anchor head of a top-loading screw is smaller than thebottom opening of the receiving member. A top-loading screw is assembledby inserting the shaft through the bottom opening, so that the anchorhead is retained within a cavity in the receiving member. Abottom-loading screw is assembled by inserting the anchor head throughthe bottom opening, and inserting and activating the securing means toprevent the anchor head from passing through the opening.

FIG. 12 illustrates an embodiment of a bottom-loading constrained motionbone screw assembly 100″ according to an alternate embodiment of theinvention. As shown, the receiving member 140″ is configured to receivea rod 12 and has a bottom opening 160″ sized and configured to allowinsertion of the anchor head 116″ therethrough. A retaining member 190is provided for retaining the anchor head 116″ within the receivingmember 140″. The illustrative retaining member 190 is disposed aroundthe bone anchor and in a groove of the receiver member 140″ to lock theanchor head 116″ within the cavity of the receiving member. As shown, acompression and restriction member 180″ comprises a capping member 181″.The capping member is shaped to accommodate the anchor head 116″.According to the illustrative embodiment, the capping member 181″includes flat surfaces 192″, 194″, that are parallel to the longitudinalaxis of the anchor shaft 118″. The anchor head 116″ includes flatsurfaces 163″, 165″ configured to mate with the flat surfaces 192″, 194″of the compression and restriction member 180″ to prevent rotation ofthe anchor about an axis that extends through the middle of the anchorhead 116″ substantially parallel to the axis of the rod, i.e., theanchor cannot move in a direction transverse to the flat surfaces 163″165″. However, the spherical shape of the side surfaces of the anchorhead adjacent to the flat surfaces allows for rotation of the anchorabout the axis T-T. After angulation of the anchor about the axis T-T bya selected amount, a closure mechanism may be inserted to lock the rodin the receiving member and/or lock the orientation of the anchor.Preferably, the closure mechanism presses down on the compression andrestriction member 180″ and locks the bone anchor between thecompression and restriction member 180″ and the retaining member 190.

Other details of the bottom-loading screw assembly 100″ shown in FIG. 12are described in U.S. Pat. No. 6,280,442, which is incorporated hereinby reference.

FIGS. 13A-C illustrate another embodiment of a constrained motion bonescrew assembly 1000 according to an alternate embodiment of theinvention. The constrained motion bone screw assembly 1000 of FIGS.13A-13C allows for pivoting of a spinal rod received in the constrainedmotion bone screw assembly 1000 in at least one direction relative tothe bone screw assembly 1000, while movement in other directions isrestricted. The illustrative constrained motion bone screw assembly 1000includes a bone anchor portion 1140, a rod-receiving portion 1400 and amovable rod seat 1800 housed by the rod-receiving portion 1400. Theillustrative rod seat 1800 includes a lower rod seat 1800 a and an upperrod seat 1800 b. The lower rod seat 1800 a and the upper rod seat 1800 bcooperate to define the rod seat 1800 for receiving the rodtherebetween. The rod seat 1800 is configured to move in one or moreselected directions relative to the bone anchor portion 1140 and therod-receiving portion 1400 to guide movement of the spinal rod 12relative to the bone screw assembly 1000. In this manner, both the boneanchor portion 1140 and the rod-receiving portion can move relative tothe spinal rod.

As shown, the bone anchor portion 1140 includes a shaft 1180 and a jointportion 1160. The joint portion 1160 includes a recess 1162 formed in atop surface thereof for receiving the lower rod seat 1800 a. The recess1162 preferably has a concave, spherical shape to allow pivoting of thelower rod seat 1800 a within the recess 1162.

The illustrative lower rod seat 1800 a, shown in detail in FIGS.14A-14B, is substantially rectangular, with a spherical bottom surface1802 configured to mate with a concave surface 1164 of the recess 1162.The lower rod seat 1800 a includes a flat top surface 1804 for seatingthe rod 12. The lower rod seat 1800 a may include retention recesses1807 a, 1807 b on opposing side surfaces 1805, 1806, respectively, forreceiving corresponding protrusions (not shown) on the rod-receivingportion 1400 to thereby couple the rod-receiving portion to the lowerrod seat 1800 a, while allowing selective relative movement between therod seat and other components of the bone screw assembly 1000.

The rod receiving portion 1400 includes a body 1402, which may compriseone or more components coupled together, and the upper rod seat 1800 bpivotably mounted to the body 1402. The upper rod seat 1800 b includes apivot point 1810 received in a recess 1403 of the body 1402 and a rodseat member 1812 connected to the pivot point 1810 via a connectingmember 1814. The recess 1403 and pivot point 1810 cooperate to allow forpivoting of the upper rod seat 1800 b in one or more selected directionsonly, while restricting pivoting in other directions.

As shown in FIG. 13C, the illustrative constrained motion bone screwassembly 1000 allows for pivoting of the rod seat 1800 relative to thebone anchor portion 1140 and/or the rod-receiving portion 1400. Theselective pivoting of the rod seat 1800 in one or more selecteddirections to allow bending of the spinal rod 12 received thereinrelative to the bone anchor portion 1140 and the rod-receiving portion1400. The lower rod seat and upper rod seat defining therebetween amovable channel for receiving the spinal rod 12 and for allowing forrelative movement between the bone anchor portion 1140 and the spinalrod 12, as well as between the rod-receiving portion 1400 and the spinalrod 12. The upper rod seat 1800 b pivots in a selected direction, andthe lower rod seat 1800 a rotates in the recess 1162 to guide the rod'smovement, while retaining the rod therebetween. The coupling between thelower rod seat 1800 a and the rod-receiving portion 1140 preventmovement in other directions. In this manner, the relative angle betweenthe rod and both the bone anchor portion 1140 and the rod-receivingportion 1400 can be selectively varied in a first direction, whilemovement in other directions is prevented.

The components of the constrained motion bone anchor assembly of theillustrative embodiments of the invention may be manufactured from anysuitable biocompatible material, including, but not limited to, metalsand metal alloys such as titanium and stainless steel, polymers and/orceramics. The components may be manufactured from the same or differentmaterials though manufacturing processes known in the art.

The present invention has been described relative to an illustrativeembodiment. Since certain changes may be made in the above constructionswithout departing from the scope of the invention, it is intended thatall matter contained in the above description or shown in theaccompanying drawings be interpreted as illustrative and not in alimiting sense.

It is also to be understood that the following claims are to cover allgeneric and specific features of the invention described herein, and allstatements of the scope of the invention which, as a matter of language,might be said to fall therebetween.

The invention claimed is:
 1. A spinal system, comprising: at least afirst bone anchor having a proximal head and a distal shaft configuredto engage bone, the proximal head having a spherical shape with opposedfirst and second non-spherical side portions; at least a first receivingmember being configured to receive the first bone anchor and having aproximal end and a distal end, the proximal end of the first receivingmember having two spaced apart upright arms defining a recess forreceiving a spinal rod therebetween, the distal end of the at least afirst receiving member having a bore therethrough; and at least a firstcap being configured to be seated within the first receiving member andhaving a proximal portion configured to seat the spinal rod, the atleast a first cap configured to extend around opposed sides of thespinal rod and above a center thereof, the at least a first cap having acavity formed in a distal portion thereof configured to at leastpartially receive the proximal head of the first bone anchor, first andsecond restrictor elements each having a first end configured to beremovably attached to the at least a first cap and a second end thatextends distally beyond the at least a first cap, and that arecomplementary to and configured to receive the first and secondnon-spherical side portions of the at least a first bone anchor; whereinwhen the first bone anchor is received in the cavity of the at least afirst cap, the first and second restrictor elements engage with at leasta portion of the first and second non-spherical side portions of theproximal head to restrict movement of the first bone anchor relative tothe at least a first cap in a direction toward and away from the sideportions of the at least a first cap.
 2. The spinal system of claim 1,wherein at least a portion of each of the first and second restrictorelements are tapered.
 3. The spinal system of claim 1, wherein at leasta portion of each of the first and second non-spherical side portions ofthe proximal head of the first bone anchor are opposed flat surfaces. 4.The spinal system of claim 3, wherein the first and second restrictorelements are configured to prevent the first bone anchor from rotatingabout an axis extending along the distal shaft of the first bone anchor.5. The spinal system of claim 3, wherein first and second inner surfacesof the first and second restrictor elements are engageable with at leasta portion of the first and second non-spherical side portions of theproximal head of the at least a first bone anchor such that the distalshaft of the bone anchor is prevented from moving about an axis that isaligned with and parallel to a longitudinal axis of a spinal rod seatedwithin the first receiving member.
 6. The spinal system of claim 1,wherein a first portion of each of the first and second non-sphericalside portions of the proximal head of the first bone anchor are opposedflat surfaces, and the cavity of the at least a first cap has a taperedshape to engage the opposed flat surfaces.
 7. The spinal system of claim1, further comprising a second bone anchor having a proximal head and adistal shaft corresponding to the at least a first anchor and a secondcap corresponding to the at least a first cap.
 8. The spinal system ofclaim 1, wherein the at least a first cap has a proximal portion with aU-shaped profile.
 9. The spinal system of claim 1, further comprising afirst closure mechanism being configured to engage with inner surfacesof the arms of the first receiving member to secure the spinal rodwithin the recess thereof.
 10. A spinal system, comprising: a pluralityof bone anchors, each of the plurality of bone anchors having a distalshaft configured to engage bone and a proximal portion engageable with aspinal rod, at least one of the plurality of bone anchors having arestrictive proximal head having a generally spherical outer surfacewith two opposed restrictive side portions interrupting the generallyspherical outer surface; at least one restrictive cap having a proximalportion configured to receive the spinal rod therein, the at least onerestrictive cap having first and second grooves in an exterior thereof,and first and second protrusions extending distally from the first andsecond grooves of the restrictive cap, the first and second protrusionsbeing configured to receive the two opposed restrictive side portions ofthe restrictive proximal head such that rotation of the correspondingbone anchor relative to the restrictive cap is restricted in at leastone dimension.
 11. The spinal system of claim 10, wherein therestrictive cap has opposed first and second at least partially curvedside portions configured to seat a portion of the restrictive proximalhead that has the generally spherical outer surface.
 12. The spinalsystem of claim 11, wherein the opposed first and second curved sideportions of the restrictive cap terminate at the two opposed restrictiveside portions of the restrictive proximal head.
 13. The spinal system ofclaim 11, wherein the first and second protrusions extend distallybetween the opposed first and second curved side portions of therestrictive cap.
 14. The spinal system of claim 11, wherein distal endsof the first and second protrusions are at least partially tapered. 15.The spinal system of claim 11, further comprising a plurality ofreceiving members, each of the plurality of receiving members beingconfigured to receive a corresponding one of the plurality of boneanchors therein and having two upright arms defining a recesstherebetween for receiving the spinal rod therein, at least one of theplurality of receiving members being configured to seat the restrictivecap therein.
 16. A surgical method, comprising: advancing a distal shaftof at least a first bone anchor into a first vertebra, the first boneanchor having a proximal head disposed in a first receiving member, thefirst receiving member having two spaced apart upright arms defining arecess for receiving a spinal rod therebetween, and having a borethrough a distal end thereof through which the distal shaft of thecorresponding first bone anchor extends; seating a first cap within thecorresponding first receiving member, the first cap having side portionsextending proximally on opposed sides of a received spinal rod above acenter thereof; and attaching first and second restrictor elements tothe first cap, each of the first and second restrictor elementsextending distally along the proximal head of the first bone anchor suchthat flat inner surfaces of the first and second restrictor elementsengage with flat side portions of the proximal head of the first boneanchor.
 17. The surgical method of claim 16, further comprising placingthe spinal rod into the recess of the first receiving member to contactthe first cap and to connect the first bone anchor with a second boneanchor to span vertebrae.
 18. The surgical method of claim 17, furthercomprising engaging a first closure mechanism with the upright arms ofthe first receiving member to secure the spinal rod therein such thatthe side portions of the first cap are secured into restrictiveengagement with the proximal head of the first bone anchor to restrictmovement of the distal shaft of the first bone anchor relative to thefirst cap in at least a first direction and to allow movement in atleast a second direction.